Protein kinases represent a large number of structurally related enzymes that are involved in signal transduction processes within cells. The basic mechanism of action of protein kinases is to add a phosphate group (i.e., phosphorylation) to a protein. Protein kinases have been classified into various groups based on the substrates phosphorylated by the protein kinase. For example, tyrosine kinase phosphorylates protein at a tyrosine residue and serine kinase phosphorylates protein at a serine residue. A large number of cellular processes and functions are regulated by protein kinases. These include proliferation, differentiation, apoptosis, gene transcription, and protein translation.
Tyrosine kinases catalyze the transfer of a terminal phosphate of adenosine triphosphate (ATP) to a tyrosine residue in the protein substrate. A kinase that phosphorylates one or more of its own tyrosines is an autophosphorylation tyrosine kinase. Tyrosine kinases can be classified as those that autophosphorylate, or those that do not autophosphorylate but that do phosphorylate other proteins. Tyrosine kinases are also classified as receptor type or non-receptor type. The receptor-type tyrosine kinases have an extracellular region, a transmembrane region, and an intracellular region. Non-receptor type tyrosine kinases are located intracellularly. There are several subgroups of the receptor-type tyrosine kinases including the group designated as HER. The HER subgroup of tyrosine kinases includes EGF receptor, HER2, HER3, and HER4 (also known as erbB-1, erbB-2, erbB-3, and erbB-4, respectively). Tyrosine kinases play an important role in both normal and abnormal cell function (Blume-Jensen et al., 2001; Andersson 2002; Levitzki 2002; Alton et al., 2002). Uncontrolled or constitutive tyrosine kinase activity can result in diseases such as cancer and immunological disorders associated with inflammatory or T-helper 1 lymphocytes (Blume-Jensen et al., 2001; Tsygankov, A. Y. 2003; Andersson 2002; Levitzki 2002; Alton et al., 2002). Many oncogenes code for proteins that are tyrosine kinases (Blume-Jensen et al., 2001).
Some growth factors and cytokines regulate cellular functions by way of the Janus Kinase (JAK) signal transducers and activators of transcription (STAT). The JAK tyrosine kinases are typically activated upon ligand binding to a receptor-type tyrosine kinase. Transcription factors (STATS) are then activated by phosphorylation. It is thought that the activated STATS are then directed to the nucleus and are subsequently involved in transcription of a target gene. The JAK family of tyrosine kinases were first described for their role in signaling through the interferon (IFN) receptors of both type I and type II IFNs (Kotenko et al., 2000). Among the IFNs, JAK2 is associated with the type II IFN, IFN-γ (Kotenko et al., 2000). The immediate-early signal transduction events associated with IFNγ's interaction with its receptor involves the obligatory action of two tyrosine kinases, JAK1 and JAK2 (Kotenko et al., 2000). The IFN-γ receptor (IFNGR) system is a heterodimeric complex consisting of an α-subunit, IFNGR-1, and a β-subunit, IFNGR-2, both of which are essential for biological activities of IFN-γ (Kotenko et al., 2000). JAK1 is constitutively associated with the IFNGR-1 chain, while JAK2 is associated with the IFNGR-2 chain (Kotenko et al., 2000).
Interaction of IFN-γ, primarily with the IFNGR-1 subunit, initiates a sequence of events that results in increased binding of JAK2 to IFNGR-1 (Kotenko et al., 2000). This interaction has important consequences for subsequent critical phosphorylation events (Kotenko et al., 2000). JAK2, in the process of binding to IFNGR-1, undergoes autophosphorylation, and at the same time IFNGR-1 is phosphorylated. These events, occurring in concert with JAK1 function, result ultimately in recruitment and tyrosine phosphorylation of the IFN-γ transcription factor STAT1α (Kotenko et al., 2000). The activity of JAK tyrosine kinases, and consequently signaling via the JAK/STAT pathway, is controlled negatively by members of the suppressors of cytokine signaling family (SOCS), also called the cytokine-inducible SH2 containing (CIS) family (Hanada et al., 2003; Kile et al., 2002; Alexander 2002; Larsen et al., 2002). These inducible proteins are of significantly varied lengths, but share domains of homology that characterize the family and their function.
As noted above, many cancer genes code for proteins that are tyrosine kinases. Because of the association of tyrosine kinases with oncogenesis and cellular proliferation, inhibitors of tyrosine kinases are being actively developed and evaluated for their use in treating various oncological disorders. The targeted approach of treatment of cancer is directed towards development of specific tyrosine kinase inhibitors. One of the most successful examples of targeted therapy against cancer is in the treatment of chronic myelogenous leukemia (CML). This form of leukemia arises from chromosomal rearrangements where the p210 BCR-Abl cytoplasmic tyrosine kinase is rendered constitutively active. The pharmaceutical compound marketed under the name GLEEVEC (imatinib mesylate) (Novartis Pharmaceutical Company, East Hanover, N.J.) binds to the ATP binding site of this kinase and inhibits its kinase activity (Capdeville et al., 2002; Levitzki 2002). This results in almost total control of CML without the undesirable side effects typically associated with conventional chemotherapy.
U.S. Pat. No. 5,912,183 (Comoglio et al.) discloses peptides which interact with intracellular signal transducers, thereby interfering in pathways associated with cell proliferation, adhesion, etc. Peptides described in the '183 patent generally contain tyrosine residues and are modeled to represent sites of tyrosine phosphorylation. Another drug (Iressa; ZD1839) that inhibits the tyrosine kinase Epidermal Growth Factor Receptor is also in clinical trials for the treatment of cancer. U.S. Pat. No. 6,417,168, published U.S. application US 2002/0165193 and published reports by Park et al. describe peptidomimetics that bind to the p185HER2/neu growth factor receptor and inhibit proliferation of p185HER2/neu overexpressing tumor cells. The p185HER2/neu protein is the human analog of p185neu. The p185HER2/neu protein is overexpressed in a significant percentage of cancers, including ovarian, breast, and colon cancer. The monoclonal antibody marketed under the name HERCEPTIN (trastuzumab) (Genentech, South San Francisco, Calif.) binds to the p185HER2/neu protein and is being used in clinical treatment of cancer (Ritter and Arteaga 2003).
However, most current therapeutics for cancer and inflammation are too nonspecific and thus not sufficiently effective. Further, chemotherapy, which is a highly non-specific treatment for cancer, is a non-targeted systemic approach to cancer treatment. As can be understood from the above, there remains a need in the art for drugs that are specifically targeted for cellular molecules that are involved or associated with inflammatory and/or oncological disorders. In particular, there remains a need for other inhibitors of tyrosine kinases, including those that are specifically inhibitory to certain kinds and classes of the tyrosine kinases.